"Straight self-defense" is arguably a bit of a misnomer given it was a fight he essentially picked, and there's the argument to be had that without Florida's SYG law, maybe Zimmerman wouldn't feel so free to play vigilante, but while I think SYG laws are terrible ideas, especially when applied as liberally as Florida's are, you're right that the Zimmerman case is not really about SYG, because if Martin was on top of Zimmerman, then he had no ability to retreat, and thus SYG is irrelevant.
In any event, I think things like SYG distract from the larger issue of the conflict between what most Americans consider acceptable behavior and the sort of behavior our gun laws actually allow.